Method of improving the north seeking result

ABSTRACT

In a process for filtering the consecutive results yi of a north-seeking gyroscope which are ooverridden by interference signals, the difference between the extreme measured values within a time window is determined. Only if this difference is less than a given maximum deviation y is the means value of the measurements in this time window given as the corrected direction of north.

PROBLEM AND PRIOR ART

North-seeking or north-determining gyros (NSG) usually serve to determine the direction of mobile equipment (vehicles, directional antennas, tunnel heading and cutting machines, geodetic instruments, etc.) and are subject to more or less heavy interference movements during the north-seeking process even if the vehicle is not moving. Methods are known (for example, from DE-OS [German Laid-Open Patent Application]3,346,450) to reduce the sensitivity to rotary interfering movements, for example with the aid of a further sensor. In real system applications, the interfering movements lie within a very broad frequency range and their amplitude often exceeds the useful signal by many orders of magnitude; however, at the same time, high measuring accuracy is required during a short measuring period (e.g., only 1 to 2 minutes)

Prior art filtering methods do not always furnish sufficiently accurate results. The measuring accuracy becomes poorer, in particular, for greater interfering movements and with increasing geographic latitude since then the usable earth rotation component decreases.

Depending on the interfering movement, a more or less noise encumbered north-seeking result is available at the output of the north-seeking gyro. The noise amplitude increases with increasing geographic latitude. The noise process is generally not stationary, that is the noise amplitude changes over time; noisy phases alternate with quieter sections, low-frequency sections alternate with higher frequency sections.

ADVANTAGES OF THE INVENTION

This signal is now subjected to digital filtering including the features of claim 1. This filter, called a "window algorithm" selects particularly the quieter phases and filters out accurate measuring results as a function of filter criteria.

The filtering method according to claim 1 can be improved further by pre-filtering methods according to claims 2 and 3. Further modifications result from the further dependent claims. The procedure results in a number of advantages, namely:

a very effective filtering out of the influences of interfering movements;

the staggering of time windows of different widths and heights permits adaptation to a very broad noise spectrum;

very easy adaptation to a specified accuracy is possible by predetermining the filter criteria (N; Δγ);

in contrast to the duration of the north-seeking process, the resulting accuracy is independent of noise spectrum and geographic latitude;

it is possible to predetermine a maximum time for the north-seeking process;

it is possible to terminate the north-seeking process if the filter criteria are not met at the end of the permissible time Tmax;

low computer expenditures;

low memory requirement in the computer;

upon termination, the filtering method can also be employed to determine a result with less accuracy;

in this process, an identification can be specified and put out which provides information with respect to the accuracy.

DESCRIPTION OF THE DRAWING FIGURES

Embodiments of the invention will be described with reference to the drawing figures.

It is shown in:

FIG. 1, the configuration of a north-seeking gyro;

FIG. 2, a block circuit diagram of the filter arrangement;

FIG. 3, diagrams;

FIG. 4, diagrams to explain the error compensation by means of the accelerometer signal;

FIG. 5, diagrams to explain the main filtering by means of the time window.

FIG. 1 shows the configuration of the gyro: a gas bearing rate gyro 1 is installed together with an accelerometer 2 in a frame 3 rotatable about the elevation axis. The elevated frame 3 can be rotated about an angle α by means of a servomotor 4, e.g. a stepping motor, so that the measuring axes of the gyro and of the accelerometer can be pivoted into discrete directions.

The arrangement is described in greater detail in DE-OS 3,346,450.

FIG. 2 is a block circuit diagram of the filter. The reference numeral 11 identifies a north-determining gyro (a gyro that can be rotated into several positions) including an evaluation circuit. Block 11 is intended to put out measurement signals (angular velocities) at given time intervals T. The reference numeral 12 identifies an accelerometer which also puts out measurement signals at time intervals T. Both signals are fed to filters 13 and 14 which filter out higher frequency noise components. The average of the pre-filtered gyro measurement values is formed in a block 15 over a time period T_(K) and this average is stored in a memory 16. At the beginning and end of period T_(K), the averages A_(a) and A_(e) of the accelerometer output signal are formed in a block 17 at intervals T_(B) and the difference (A_(e) -A_(a)) is formed by means of a memory 18 and a difference former 19.

After division by the gravitational acceleration g and the measuring interval T_(K) in a block 20, the rate of tilt change determined by accelerometer 12 during the time T_(K) is obtained as follows: ##EQU1##

This result is stored in a memory 21. In a difference former 22, the difference is formed between the average Ω of the gyro signal stored in memory 16 and the rate of tilt change Δν stored in memory 21. The signal corrected by Δν is then divided in a block 23 by the horizontal component Ω_(H) = ω_(E) cosφ of the earth rotation vector. The result is the north-seeking result γi at time i, which is fed into a memory 24. This and adjacent north-seeking results are then filtered once more in a main filter 25 equipped with a time window. The filter criteria (number of north-seeking results γi and maximum deviations Δγ included in the time window) can be varied by way of an input 26.

FIG. 3 shows the output signals of the gyro and of the accelerometer. FIG. 3 shows a gyro signal including only little noise; its average over the time period T_(K) is Ω_(R) (average at rest). FIG. 3b shows a gyro signal in which a greater interfering movement occurs in its middle section. This results in an average Ω_(S) and an average offset ΔΩ. This offset results in a north-determination error δγ as follows:

    δγ=Δω/ωH

The accelerometer simultaneously senses a change in tilt

    Δν=ν.sub.e -ν.sub.a

about the gyro measuring axis, where

ν_(a) =A_(a) /g and ν_(e) =A_(e) /g (see FIG. 3c).

The noise angle velocity is calculated from the change in tilt as

    ΔΩ.sub.S =Δν/T.sub.K

where

Ω_(R) =the average over time of the non-interfered-with gyro signal over the time period T_(K) ;

Ω_(S) =the average over time of the interfered-with gyro signal over the time period T_(K) ;

ω_(E) =the angular velocity of the earth's rotation;

φ=the geographic latitude;

ν_(a) =the tilt angle at the beginning of measuring interval T_(K) ;

ν_(e) =the tilt angle at the end of measuring interval T_(K) ;

A_(a) =the average of the accelerometer signal over a time period T_(B) at the beginning of measuring interval T_(K) ;

A_(e) =the average of the accelerometer signal over a time period T_(B) at the end of measuring interval T_(K) ;

g=the gravitational acceleration.

FIG. 4 is a schematic representation of the filter (15-22) which utilizes the accelerometer signal: the measurement begins at time t₀ =0 with the formation of an average of the accelerometer signal over a time period T_(B) (17). The result A_(a0) is stored (18). At time T_(B) /2, averaging of the gyro signal begins over a time period T_(K) (15). The result Ω_(S0) is stored (16). At time t=T_(K),averaging of the acceleration signal begins, again over a time period T_(B) (17). The result A_(e0) is also stored. On the basis of the above formulas, the north-seeking result γ₀, which has been corrected with respect to the interfering rotational movements, is now calculated (19-23) from the stored averages.

The described sequence is repeated at discrete time intervals Δ t (e.g., 1 second). Thus, the north-seeking process started, for example, at time t_(i), that is, the sequence offset in time by t_(i), furnishes the north-seeking result γ_(i). All intermediate results γ_(i) are stored. This results in the curve for the intermediate north-seeking result γ which is shown in the lower portion of FIG. 4 (and still contains a lot of noise). Its average (averaged over a long period of time) is the desired north-seeking result γ_(m).

FIG. 5 shows the main filtering of the intermediate north-seeking result with the aid of a time window: here, beginning at time T₀ =t₀ +T_(B) +T_(K), the minimum values γmin and the maximum values γmax are each determined from a number N of intermediate results. Then a check is made whether the difference is

    γmax-γmin<Δγ

where Δγ, the permissible maximum deviation ("window height"), and N, the number of measurements under consideration ("window width"), are the two filter criteria. If this condition is not met, the process is offset in time by Δt, that is, it is repeated beginning at time T₀ +Δt until either

(a) the above filter criterion has been met; or

(b) the maximum permissible measuring duration Tmax has expired.

In case (a), the north-seeking result is now obtained by forming an average from the N intermediate results for which the criterion has been met.

In case (b), the north-seeking process is terminated.

Once the measurement values are stored, a new passage with reduced filter criteria can be started at the end of Tmax and this can be repeated until a result is available. In this case, the result is advisably identified with respect to the filter criteria employed so as to obtain information about its (reduced) accuracy.

The drawing figures served to describe the case where one time window is employed. However, it is also possible to employ several time windows simultaneously with different filter criteria and to form the north-seeking result from the average of that window for which the filter criterion was met first. The number of north-seeking results N and the maximum deviations Δγ can be selected in such a manner that the same accuracies result in spite of different criteria.

It is possible to have the user determine the criteria individually, or groups of criteria N, Δγ which each furnish the same accuracy can be stored in a memory and the user can select the group representing the desired accuracy. However, it is also possible to predetermine windows which, with increasing measuring duration, permit greater errors. It is favorable if the various window widths are selected in such a way that N is of a ratio of 1:2:. . .:2.sup.(n-1). 

We claim:
 1. A method of improving the north-seeking result determined by means of a north-seeking gyro, wherein the determined north-seeking results γi are put out at given time intervals and an average is formed from a plurality (N) of successive north-seeking results lying within a time window, characterized in that initially the maximum north-seeking result γmax and the minimum north-seeking result γmin within the time window are determined and the average γ_(m) is put out as the north-seeking result to be employed only if the difference γmax-γmin<Δγ, where Δγ constitutes a given maximum deviation (filter criteria).
 2. A method according to claim 1, characterized in that the higher frequency noise components are filtered out of the north-seeking results put out by the north-seeking gyro.
 3. A method according to claim 1, characterized in that an accelerometer is also accommodated in a frame that is rotatable about the elevation axis and in which the north-seeking gyro is mounted, and the following method steps are employed to filter out low frequency noise components:the put-out gyro measurement values are averaged over a given time period T_(K) ; the change Δν of the tilt angle is determined over the time period T_(K) ; the velocity of the change Δν of tilt is determined by dividing it by the period duration T_(K) ; the difference is formed between the gyro average and the rate of tilt angle change; the resulting difference is divided by the horizontal component Ω_(H) of the earth's rotation and is processed further each time as the put-out north-seeking result.
 4. A method according to claim 1, characterized in that the number (N) of the north-seeking results lying within the time window is adjustable.
 5. A method according to claim 1, characterized in that the maximum deviation Δγ is adjustable.
 6. A method according to claim 1, characterized in that a plurality of time windows including a different number of north-seeking results (N) and a different maximum deviation Δγ are employed simultaneously and the north-seeking result is formed from the average of that window for which the respective filter criterion is met first.
 7. A method according to claim 6, characterized in that the number of north-seeking results N and the maximum deviations Δγ are selected in such a way that they furnish approximately the same accuracy.
 8. A method according to claim 6, characterized in that the filter criteria are put in individually by the user.
 9. A method according to claim 6, characterized in that the user is able to call up the filter criteria in groups from a memory, with each individual group being a measure of the desired accuracy.
 10. A method according to claim 6, characterized in that, with an increasing number of north-seeking results (N), the maximum deviations Δγ are selected in such a manner that with increasing measuring duration, a greater measuring error is permissible.
 11. A method according to claim 6, characterized in that the number of north seeking results (N) is of a ratio as 1:2:. . .:2.sup.(n-1).
 12. A method according to claim 1, characterized in that, upon a determination of γmax-γmin>Δγ, a new determination of the north-seeking result is initiated.
 13. A method according to claim 12, characterized in that the north-seeking process is terminated after a maximum time period Tmax without a usable north-seeking result.
 14. A method according to claim 13, characterized in that, if all determined north-seeking results γi are stored, the results γi are filtered again after the north-seeking process has been terminated and are possibly filtered repeatedly with step-wise increasing maximum deviation Δγ; and that average north-seeking result γ_(m) is put out whose individual results met the existing filter criterion first.
 15. A method according to claim 14, characterized in that the put-out result is identified with respect to the step at which it met the filter criteria.
 16. A method according to claim 3, characterized in that the rate of change Δν of the tilt angle Δν {sic} is obtained from the difference between the acceleration signals at the beginning and end of the period T_(K) and multiplication with ##EQU2##
 17. A method according to claim 16, characterized in that the accelerometer signals are averaged for shorter periods of time T_(B) at the beginning and end of the period T_(K).
 18. A method according to claim 3, characterized in that the filter parameters N, Δγ and Tmax are permanently stored as instrument constants. 